In recent years, as the awareness on environmental issues has grown in the automotive field, exhaust emission regulations have become more stringent and countermeasures have been underway to suppress carbon dioxide gas emission.
For example, in addition to the countermeasures such as weight reduction or the installation of exhaust gas treatment devices such as an Exhaust Gas Recirculation (EGR), a Diesel Particulate Filter (DPF), a urea Selective Catalytic Reduction (SCR) system or the like, countermeasures such as the use of fuels, for example, bioethanol, biodiesel fuel or the like have been in practice.
Bioethanol is an ethanol produced from biomass, and is mixed with gasoline to be used as a fuel for a gasoline engine. Biodiesel fuel is a fuel obtained by mixing fatty acid methyl ester with a diesel fuel and used as a fuel for a diesel engine. Herein, ethanol is produced from raw materials such as corn or sugar cane. Fatty acid methyl ester is produced by esterifying raw materials such as plant oils or waste oils, and examples of the plant oils include rapeseed oil, soybean oil, coconut oil and the like.
Biofuels such as bioethanol or biodiesel fuel have a high corrosiveness to metal materials compared to typical fuels. When a biofuel is used, the effects of the biofuel are examined in advance on the in-use performance of various members which configure fuel system parts. The needs for materials with a higher reliability have been requested from manufacturers who commit themselves to ensure an ultra-long use-life of parts, and stainless steel is one of the candidate materials.
The following technologies are known as the related arts where stainless steel is used for a fuel tank or a fuel supply tube among fuel system parts.
Patent Document 1 discloses a ferritic stainless steel sheet which contains, by mass %, C, 0.015% or less, Si: 0.5% or less, Cr: 11.0% to 25.0%, N, 0.020% or less, Ti: 0.05% to 0.50%, Nb: 0.10% to 0.50% and B: 0.0100% or less, and, as necessary, further contains, by mass %, one or more elements selected from among Mo: 3.0% or less, Ni: 2.0% or less, Cu: 2.0% or less and Al: 4.0% or less. The total elongation of the steel sheet is in a range of 30% or more, and the Lankford value thereof is in a range of 1.3 or more.
Patent Document 2 discloses a ferritic stainless steel sheet which contains, by mass %, C, 0.01% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.06% or less, S: 0.03% or less, Cr: 11% to 23%, Ni: 2.0% or less, Mo: 0.5% to 3.0%, Al: 1.0% or less and N, 0.04% or less, and satisfies a relational expression of Cr+3.3Mo≧18. The steel sheet further contains either one or both of 0.8% or less of Nb and 1.0% or less of Ti and satisfies a relational expression of 18≦Nb/(C+N)+2Ti/(C+N)≦60. The grain size number of the ferrite crystal grains of the steel sheet is in a range of 6.0 or more, and an average r-value is in a range of 2.0 or more.
Patent Document 3 discloses a ferritic stainless steel sheet which contains, by mass %, C, 0.01% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.06% or less, S: 0.03% or less, Al: 1.0% or less, Cr: 11% to 20%, Ni: 2.0% or less, Mo: 0.5% to 3.0%, V: 0.02% to 1.0% and N: 0.04% or less, and contains, by mass %, either one or both of 0.01% to 0.8% of Nb and 0.01% to 1.0% of Ti. When the steel sheet is subjected to a uniaxial tension and deformed by 25%, the height of an undulation generated on the surface is in a range of 50 μm or less.
However, the technologies in Patent Documents 1 to 3 deal with corrosion resistance against typical gasoline. As described below, since corrosiveness of biofuels was greatly different from corrosiveness of gasoline, the technologies were not sufficient enough to deal with corrosion resistance against the biofuels.
In addition, in the related art, it is hard to say that corrosiveness of biofuels to stainless steel are necessarily made clear in detail, and that corrosion resistance of various stainless steels against biofuels is necessarily made clear.
In addition to the above-described countermeasures on fuel as countermeasures for environmental issues in the automotive field, a countermeasure is launched to improve fuel economy by mounting a heat exchanger recovering exhaust heat, a so-called exhaust heat recovery unit on hybrid vehicles. The exhaust heat recovery unit is a system where exhaust gas heats engine coolant and the heated engine coolant is used for a heater or the warm-up of an engine, and is also called as an exhaust heat recirculation system. Accordingly, the exhaust heat recovery unit shortens a time from cold start to engine stop in hybrid vehicles, and contributes to improvement in fuel economy particularly in the winter season.
A heat exchange section of an exhaust heat recovery unit is required to have a good thermal conductivity to obtain a good thermal efficiency. In addition, since a heat exchange section is in contact with exhaust gas, the heat exchange section is required to have excellent corrosion resistance against condensate water in exhaust gas. On the other hand, the exterior of the exhaust heat recovery unit is also required to have excellent corrosion resistance against salt damage. Such a corrosion resistance is required even for members in the downstream of an exhaust system where a muffler is a main body. However, since there is a concern that the corrosion in the exhaust heat recovery unit results in a serious accident such as the leakage of coolant, the exhaust heat recovery unit is required to have greater safety and better corrosion resistance.
In the related art, ferritic stainless steels such as SUS430LX, SUS436JIL and SUS436L containing 17% or more of Cr are used for portions where corrosion resistance is particularly required among members in the downstream of an exhaust system where a muffler is a main body. The material of an exhaust heat recovery unit is required to have corrosion resistance equal to or higher than corrosion resistance of these stainless steels.
In addition, since the structure of a heat exchange section is complicated, the heat exchange section is fabricated not only by welding but also by brazing. The material of a heat exchange section fabricated by brazing is required to have good brazeability. Furthermore, since an exhaust heat recovery unit is installed in the downstream of an underfloor catalytic converter in many cases, the temperature of exhaust gas becomes high at the inlet of the exhaust heat recovery unit. In addition, exhaust gas is forcibly cooled by heat exchange. Therefore, the exhaust heat recovery unit is required to have good thermal fatigue characteristics.
Patent Document 4 discloses an automotive exhaust heat recovery device made of a ferritic stainless steel. The ferritic stainless steel contains C, 0.020% or less, Si: 0.05% to 0.70%, Mn: 0.05% to 0.70%, P: 0.045% or less, S: 0.005% or less, Ni: 0.70% or less, Cr: 18.00% to 25.50%, Cu: 0.70% or less, Mo: 2/(Cr-17.00) % to 2.50% and N: 0.020% or less. The ferritic stainless steel further contains either one or both of 0.50% or less of Ti and 0.50% or less of Nb and satisfies a relational expression of (Ti+Nb)≧(7×(C+N)+0.05), and the remainder thereof is Fe and unavoidable impurities. In the ferritic stainless steel according to Patent Document 4, Mo is added together with 18% or more of Cr; and thereby, corrosion resistance against condensate water in exhaust gas is ensured.
Patent Document 5 discloses a ferritic stainless steel sheet which contains C, 0.05% or less, Si: 0.02% to 1.0%, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, Cr: 20% to 25%, Cu: 0.3% to 1.0%, Ni: 0.1% to 3.0%, Nb: 0.2% to 0.6% and N: 0.05% or less, and has excellent crevice corrosion resistance. The steel sheet includes Nb carbonitrides having sizes of 5 μm or smaller, and the surface roughness Ra of the steel sheet is in a range of 0.4 μm or smaller. In the ferritic stainless steel sheet according to Patent Document 5, both of Ni and Cu are added together with 20% or more of Cr; and thereby, crevice corrosion resistance is ensured.
Patent Document 6 discloses an automotive exhaust gas passage member made of a ferritic stainless steel. The ferritic stainless steel contains C, 0.015% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.045% or less, S: 0.010% or less, Cr: 16% to 25%, Nb: 0.05% to 0.2%, Ti: 0.05% to 0.5%, N, 0.025% or less and Al: 0.02% to 1.0%. The steel further contains either one or both of 0.1% to 2.0% of Ni and 0.1% to 1.0% of Cu at a total content (Ni+Cu) of 0.6% or more. In the ferritic stainless steel sheet according to Patent Document 6, Ni and Cu are added at a total content of 0.6% or more; and thereby, good corrosion resistance is achieved at a low cost without the use of expensive Mo.
Patent Document 7 discloses a stainless steel for a heat pipe of a high-temperature exhaust heat recovery device which contains Cr: 16% to 30%, Ni: 7% to 20%, C, 0.08% or less, N, 0.15% or less, Mn: 0.1% to 3%, S: 0.008% or less and Si: 0.1% to 5%, and satisfies Cr+1.5Si≧1 and 0.009Ni+0.014Mo+0.005Cu−(0.085Si+0.008Cr+0.003Mn)≦−0.25. A technology according to Patent Document 7 relates to not a heat exchanger where heat is exchanged between exhaust heat and coolant, but an exhaust heat recovery unit using a heat transmission device which is called as a heat pipe. Patent Document 7 discloses an austenitic stainless steel suitable for the heat pipe.
An exhaust heat recovery unit is required to have corrosion resistance equal to or higher than corrosion resistance of a ferritic stainless steel containing 17% or more of Cr. However, in a ferritic stainless steel containing 17% or more of Cr in the related art, corrosion resistance after brazing was not considered. For this reason, when the existing ferritic stainless steel was used for an exhaust heat recovery unit, corrosion resistance after brazing could not be sufficiently ensured due to a change in the metallographic texture of a brazed portion or the progress of oxidation of the steel surface.